1. Field of the Invention
The present invention relates to an image display apparatus that combines light fluxes from plural display elements to present one enlarged combined image, in particular, relates to an image display apparatus suitable for a head-mounted display (HMD).
2. Description of the Related Art
For observation optical systems used for image display apparatuses such as HMDs, image presentation with a wide view angle is required to enable natural observation of images and to increase realistic sensation. Moreover, it is desirable that such image display apparatuses being mounted on an observer's head be small and thin.
Some of such small and thin image display apparatuses capable of presenting images with a wide view angle use plural display elements displaying original images for mutually different view angles, and form one image by combining plural enlarged images corresponding to the original images to enable observation of an enlarged combined image. The use of the plural display elements enables reduction in size and thickness of an optical system for each of the plural display elements, which realizes a wholly small and thin image display apparatus capable of presenting images with a wide view angle.
Such image display apparatuses have been disclosed in Japanese Patent Laid-Open Nos. 07-274097, 11-326820, 10-246865 and 09-166759 and Japanese Patent No. 3524569.
The image display apparatuses disclosed in Japanese Patent Laid-Open Nos. 07-274097, 11-326820 and 10-246865 and Japanese Patent No. 3524569 use an optical system incapable of overlapping light fluxes reaching a boundary part of images mutually combined. Therefore, when an observer's eye is rotated to generate a gap between an exit pupil of the optical system and an observer's pupil (in other words, between a visual axis of the image display apparatus and the observer's eye), a light flux from part of the combined image is not introduced to the observer's pupil, and thereby image lacking is observed by the observer.
FIG. 22A shows a state where an observer's eye 7 is rotated in a conventional image display apparatus whose optical system is constituted by a V-shaped mirror 203 and an ocular lens 204. When the observer's eye 7 is rotated upward, a light flux from a display element 202 forming a lower side shallow view angle is introduced to the eye (pupil) 7, but a light flux from a display element 201 is not introduced thereto. Therefore, only the light flux forming the lower side shallow view angle is introduced to the eye (pupil) 7.
To solve such a problem, as shown in FIG. 22B, Japanese Patent No. 3524569 has disclosed a method of enlarging a display area of each of display elements 205 and 206 in a divisional direction of a combined image and providing to original images displayed on the display elements 205 and 206 overlapping areas where a same original image is displayed. Such provision of the overlapping areas causes the light flux that is not introduced to the eye (pupil) in the image display apparatus shown in FIG. 22A is introduced thereto from the display element 205.
However, it is necessary for introducing light flux from all the original images to the eye (pupil) to enlarge the overlapping area. For example, in a case where a horizontal view angle is 50 degrees, a vertical view angle is 40 degrees, a diameter of an exit pupil of the optical system is 12 mm, an eye relief (distance from the exit pupil to the optical system) is 20 mm and the horizontal view angle is divided into plural view angles by the method disclosed in Japanese Patent No. 3524569, it is necessary for introducing the light fluxes from all the original images to the eye (pupil) to cause each display element to display an original image corresponding to a view angle of 38.5 degrees that is a sum of half (25 degrees) of the horizontal view angle (50 degrees) and a view angle of the overlapping area (13.5 degrees).
In other words, the view angle of the overlapping area is 27 degrees with respect to the entire horizontal view angle of 50 degrees, which is a larger view angle than half of the entire horizontal view angle. Such a large overlapping area makes image information presentation efficiency of each display element low, which makes it difficult to achieve an image display apparatus capable of presenting images with a wide view angle.
Moreover, the apparatus disclosed in Japanese Patent Laid-Open No. 9-166759 uses in a first ocular optical system a concave mirror formed as a half-mirror to prevent a phenomenon of image darkening or image lacking occurring near a center of a displayed image due to a joint line (ridge line) of two mirrors disclosed in Japanese Patent Laid-Open No. 7-274097 and Japanese Patent No. 3524569.
However, the apparatus disclosed in Japanese Patent Laid-Open No. 9-166759 prevents such image lacking occurring at a joint line of an enlarged image when the visual axis of the observer does not match the visual axis of the apparatus by providing overlapping areas in two original images where a same partial image is displayed. Thus, Japanese Patent Laid-Open No. 9-166759 has not mentioned a case where there is no overlapping area in the two original images.
Moreover, the apparatus disclosed in Japanese Patent Laid-Open No. 9-166759, all of light from a prism disposed far from an observer passes through a prism disposed near the observer. Therefore, a size of the prism disposed near the observer is increased, and thereby a size of the entire optical system is increased, which is not suitable for a small and thin HMD.
Specifically, the size of the optical system in the apparatus disclosed in Japanese Patent Laid-Open No. 9-166759 is increased in the following two directions. In a horizontal direction, all of the light that has emerged from one of the display elements and then has passed through the prism disposed far from the observer passes through the prism disposed near the observer. Therefore, the horizontal size of the prism disposed near the observer is increased for an effective light flux from the other display element. Moreover, since the light is introduced in a vertical direction as well as in the horizontal direction, the prism disposed far from the observer and the prism disposed near the observer are arranged so as to nearly entirely overlap with each other in the vertical direction, which makes the vertical size (thickness) of the optical system larger.